The molecular weights and the integrity of the two major components of ribosomal RNA from a wide variety of eukaryotic species, from protozoa to man, has been investigated by polyacrylamide gel electrophoresis under fully denaturing conditions. The results show that the s-rRNA is largely heterogeneous, ranging in size, from 0.65 X 10(6) to 0.96 X 10(6) dalton. The l-rRNA ranges in size from a minimum mol wt of 1.28 X 10(6) to a maximum weight of 1.60--1.66 X 10(6) (of warm-blooded vertebrates, Cephalopoda and Diptera); several intermediate values have been found in lower organisms and Protozoa. The s-rRNA is a truly continous, uninterrupted polynucleotide chain in all groups of organisms (protozoans, plants, fungi and animals). The larger rRNA is a continous un-nicked chain in all of the deuterostomian animals, plants and fungi. However, the l-rRNA of all the protostomian animals and the protozoa is an aggregate molecule consisting of two subunits held together by limited regions of hydrogen bounding; in these organisms the size of the s-rRNA is generally identical to that of the larger fragment of the l-rRNA. Analysis of the molecular weights of the subunits of the l-rRNA in the protostomians and the protozoa suggests that the l-rRNA contains one short stretch, prone to nucleolytic attack, dividing the RNA chain into a molecularly conserved portion (0.65 to 0.72 X 10(6) dalton) and a variable portion (0.65 to 0.96 X 10(6) dalton).